Francis Alys’ Sometimes Making Something Leads to Nothing (or Paradox of Praxis I) shows the artist pushing a large, and later smaller, block of ice around the streets and sidewalks of Mexico City. As he pushes the block of ice, the ice melts slowly and leaves a trail of water behind him that eventually evaporates leaving no trace that he was actually there. As the block becomes smaller, it becomes easier to push around the streets. At the end of his project, no impact of what Alys has done is left behind: the block of ice has melted into the evaporated water trail. The only evidence that the artist did anything is in the process of the project that was captured on video. Alys left a temporary and non-harmful impact on his environment where the effort used to do it was very disproportionate from the result.
http://www.youtube.com/watch?v=QbvFQj-INYk